The present invention relates to a semiconductor laser module having a temperature adjustment function suitable for optical communications, optical information processing, and the like.
A conventional semiconductor laser module of this type is used as a signal source of an optical fiber transmission unit or the like. The conventional semiconductor laser module is arranged as shown in FIG. 3. Referring to FIG. 3, reference numeral 1 denotes a semiconductor laser; 2, a lens for focusing a laser beam from the semiconductor laser 1; and 3, an optical fiber for optically coupling the beam emitted from the semiconductor laser 1 and focused and emerging from the lens 2. The laser beam emitted from the semiconductor laser 1 is focused on the optical fiber 3 through the lens 2. The semiconductor laser 1, the lens 2, and the optical fiber 3 are arranged on a line so as to optimize coupling efficiency.
Reference numeral 4 denotes a convex board for fixing the semiconductor laser 1, the lens 2, and the like; 5, an electronic cooling element for adjusting the temperature of the semiconductor laser 1; and 6, a module package incorporating the convex board 4, the electronic cooling element 5, and the like and fixing the optical fiber 3. The semiconductor laser 1 is mounted on a heat sink 11. The heat sink 11 is soldered and fixed on the upper surface of the convex board 4. A lens holder 7 for fixing the lens 2 is welded and fixed on the upper surface of the convex board 4 by a YAG (Yttrium Aluminum Garnet) laser. The convex board 4 is soldered and fixed on the upper surface of the electronic cooling element 5. The electronic cooling element 5 is soldered and fixed to the bottom surface in the module package 6. The optical fiber 3 is welded and fixed to a fiber holder 10 by the YAG laser. The fiber holder 10 is welded and fixed to the module package 6 by the YAG laser.
In recent years, in an optical fiber transmission unit, strong demand has arisen for a low-profile semiconductor laser module as a signal source to attain a high packaging density. As described above, in the conventional semiconductor module, the semiconductor laser 1, the lens 2, and the optical fiber 3 are arranged on a line, and at the same time, the semiconductor laser 1 and the lens 2 are mounted on the electronic cooling element 5. For this reason, the height of the module package 6 is determined by the heights of the lens 2, the convex board 4, the electronic cooling element 5, and the like which are constituent components of the module package 6.
As one method of reducing the height of the module package 6, the constituent components of the module package 6 must be made compact.
There are limitations in making the constituent components compact. For example, to efficiently couple a laser beam emitted from the semiconductor laser 1 to the optical fiber 3 focused by the lens 2, the NA (Numerical Aperture) for the laser beam incident on the lens 2 must be increased. A decrease in lens diameter while the NA is kept large has physical limitations. In the convex board 4, the strength and thickness of a portion on which the lens holder 7 is disposed have a lower limitation. This portion must have a predetermined thickness or more. In addition, when the height of the electronic cooling element 5 is reduced, the cooling capacity is decreased.
As described above, in the conventional semiconductor laser module, its height cannot be reduced due to the dimensional limitations of the lens 2, the lens holder 7, the convex board 4, and the electronic cooling element 5, which are the constituent components of the laser module, and due to the mounting structure in which these components are mounted on the electronic cooling element 5.
As a conventional low-profile photosemiconductor module, for example, Japanese Patent Laid-Open No. 5-226779 proposes a low-profile structure in which a temperature adjustment electronic cooling element (Peltier cooler element) is directly assembled on the bottom surface of a package to eliminate a board below the Peltier cooler element, thereby reducing the thickness of the board portion. Japanese Patent Laid-Open No. 5-150146 proposes a low-profile, deformation-resistant structure in which thick-wall portions are disposed at both sides of a board in a direction parallel to an optical axis to reduce the thickness of the board without degrading the bending stress in the optical axis. Still another conventional structure is proposed in which a groove is formed in the bottom portion of a package to reduce the total thickness of an electronic cooling element and the components of an optical coupling system, all of which are disposed in the groove, thereby obtaining a low-profile module. In all the conventional structures, as shown in FIG. 3, a lens holder is fixed on a board. Therefore, the problem of effectively reducing overall module height is left unsolved.